Fix siginfo_t.si_status for wait6/waitid/SIGCHLD.

Per POSIX, si_status should contain the value passed to exit() for
si_code==CLD_EXITED and the signal number for other si_code. This was
incorrect for CLD_EXITED and CLD_DUMPED.

This is still not fully POSIX-compliant (Austin group issue #594 says that
the full value passed to exit() shall be returned via si_status, not just
the low 8 bits) but is sufficient for a si_status-related test in libnih
(upstart, Debian/kFreeBSD).

PR:		kern/184002
Reported by:	Dmitrijs Ledkovs
Tested by:	Dmitrijs Ledkovs